Fired heaters or furnaces, herein used interchangeably, are common elements of industrial plants. With reference to the reformer furnace R illustrated in FIG. 1, a section F (or sections) typically referred to as the radiant section or firebox contains a means of oxidizing fuel to release heat and reaction products (“flue gas” or “exhaust gas”). This is commonly achieved by combustion of fuel and air using industrial burners B, and in the case of heating and/or reacting fluids, the radiant section F contains a plurality of tubes T to heat the fluids.
Furnaces include reformers, steam crackers, other reactors, and non-reactive heaters employing burners or other oxidative methods of generating heat and creating flue gas. As used herein, flue gas encompasses any combination of combustion products or effluent gas.
Whether the radiant section F is used for reacting components or merely for heating, the flue gas is gathered in a series of longitudinal tunnels 1 including opposite outside row tunnels 2 and one or more inside row tunnels 4, and passed through transition section 6 to convection section 8, which is a heat recovery section dominated by convective heat transfer where additional heat is often recovered from the flue gas.
With reference to some additional details shown in FIGS. 2 and 3, the flue gas flows into the tunnels 1 through uniformly sized openings or ports 10 spaced along the length of the tunnels, and exits from the open ends 12 into the transition section 6 and/or convection section 8, or other downstream flue gas processing equipment.
The tunnels 1 may include a pair of side walls 16 in each inside tunnel, or a side wall 18 in the case of an outside tunnel, and an outer wall 20 against the firebox wall 22 in the case of the outside row tunnels 2, which are erected from the furnace floor 24 using insulating firebricks transitioning to regular firebrick secured with mortar up to a roof or lid 26, sometimes called coffin covers, often made from large refractory slabs with large expansion gaps created at regular intervals to account for thermal expansion.
The tunnels 1 have flow channels for the flue gas which are normally of uniform width, height and cross-sectional area between the open end 12 and closed end 14. To balance the amount of flue gas entering the tunnels 1 at various points along their length, the number of openings or ports 10 per interval is decreased relative to the pressure drop through the ports 10, which due to the velocity of the flue gas in the tunnels 1, usually means that the number of holes is decreased relative to the distance of the interval from the closed end 14 of the tunnel 1, or stated another way, increased relative to the distance of the interval from the exit end 12 of the tunnel. The ports 10 are formed as the walls 16, 18 are constructed by leaving out half blocks in regular patterns. Since the outside row tunnels 2 generally receive flue gas from the outside row of burners from one side only, these tunnels are usually sized to receive only a fraction of the flue gas passing through the inside row tunnels 4, e.g., 65%. An example applying the industry standard design principles for flue gas tunnels is presented in Table 1.
TABLE 1Example of Flue Gas Tunnel Design Principles*PropertyClosed EndMidpointOpen EndInside Tunnel Velocity, 015 (50)  30 (100)m/s (ft/sec)Total Gauge Pressure,−187 (−0.75)−189 (−0.76)−194 (−0.78)Pa (in. w.c.)Velocity Pressure, 0 72 (0.29)291 (1.17)Pa (in. w.c.)Static Gauge Pressure,−187 (−0.75)−261 (−1.06)−490 (−1.95)Pa (in. w.c.)Firebox Gauge Pressure,−150 (−0.60)−150 (−0.60)−150 (−0.60)Pa (in. w.c.)Differential Pressure, 37 (0.15)114 (0.46)336 (1.35)Pa (in. w.c.)Calculated Relative Opening105.73.4Area (dimensionless)Actual Relative Opening106  3  Area Due to Rounding toNumber of Openings*From BD Energy Systems Steam Methane Reformer Advanced Training Course Handbook, Part 2 - Critical Design Features, Chapter 4 - Radiant Section (2015). Based on tunnels of uniform cross-sectional flow area.
From this example, it is seen that the use of uniform opening sizes only allows a rough approximation of the desired opening area in each interval. Furthermore, because of the high temperature and thermal stresses, the tunnels 1 are usually constructed with pilasters in the walls at regular intervals 28, which do not allow the placement of openings using conventional block construction techniques. The placement of ports 10 in the industry standard tunnel design thus usually results in a very uneven, fluctuating entry or mass flow rate of flue gas along the various intervals, as shown in FIG. 4. Moreover, the ports 10 are normally positioned near or upwardly from the bottom of the tunnels 1, so that there is greater flue gas flow at the bottom of the tubes T especially where they are more vulnerable to these excessive temperature fluctuations. While heating in the firebox F is dominated by radiance, the temperature fluctuations can be sufficiently substantial, especially during startup and/or shutdown, to eventually result in premature tube failure and loss of the flow of reactants through the failed tubes, which in turn further exacerbating the temperature fluctuations.
Other efforts to make the flow of flue gas into the tunnels more uniform have included angled slots in the lid of the tunnel and an increasing cross-sectional area of the tunnel to maintain a uniform velocity of flue gas in the tunnel, as described in US 2007/0234974 A1.
Additional design parameters and issues are described in BD Energy Systems Steam Methane Reformer Advanced Training Course Handbook, Part 2—Critical Design Features, Chapter 4—Radiant Section (2015), a copy of which is appended hereto and made a part hereof in its entirety.
Recently, stackable, interlocking refractory blocks made with mullite and/or alumina resistant to high temperature creep, have been made available to the industry, such as those described in US 2006/0242914 A1; or those described in J. Quntiliana et al., “Improving Flue Gas Tunnel Reliability, Nitrogen+Syngas, No. 336, p. 59 (July-August 2015), and WO 2015/188030, e.g., the STABLOX™ flue gas tunnel system commercialized by Blasch Precision Ceramics (Albany, N.Y.). The use of these tunnel systems has facilitated a more versatile location of the ports 10, as well as a more stable and quicker tunnel wall construction. Even so, optimizing the industry standard tunnel design for more precise placement of uniformly sized ports 10, still results in a significant variation in flue gas mass flow rates, e.g., as seen in the example of FIG. 5.
The industry would benefit from improved flue gas tunnel designs and operations that avoid or lessen the extent of drawbacks associated with the fluctuation of flue gas flow into and/or within the flue gas tunnels.